I. Field of the Invention
The present invention relates generally to opto-electronic position detection systems for providing information regarding the three-dimensional alignment status of an ophthalmic instrument relative to an eye of a patient, and more particularly to an afocal position detection system capable of providing updated position information at a very high rate of repetition.
II. Description of the Related Art
Opto-electronic position detection and alignment systems for use in locating an ophthalmic instrument relative to an eye to be tested are well known, as evidenced by U.S. Pat. No. 4,881,807 to Luce et al. Where the ophthalmic instrument is a non-contact tonometer having a discharge tube for directing a fluid pulse at the eye, X-Y alignment is typically achieved by aligning an axis of the discharge tube to intersect with the corneal vertex, and Z alignment is achieved by positioning a fluid exit end of the discharge tube at a predetermined distance from the corneal vertex. In systems of the prior art, triangulation is used to gauge the three-dimensional location of the eye relative to the instrument. By way of example, the aforementioned U.S. Pat. No. 4,881,807 discloses a system wherein two light sources arranged on opposite sides of the eye illuminate the eye with divergent rays, and a pair of CCD area detectors each comprising a two-dimensional array of light-sensitive pixels are arranged behind associated pinhole apertures to receive a small bundle of reflected rays originating from a corresponding one of the light sources. A local x-y location where the light strikes the CCD array is determined by identifying the pixel registering the peak response signal. The local x-y locations where light strikes each CCD array and specifications describing the predetermined geometric arrangement of the system components are provided as inputs to a microprocessor, which then calculates the amount of movement in the global X, Y, and Z directions necessary to achieve alignment.
While the system described above has proved suitable for use in large table mounted instruments where the patient""s head is stabilized by a forehead rest to minimize inadvertent relative movement between the instrument and the eye, the fact that a large number of CCD pixels must be scanned to make a single position determination introduces a limitation as to the repetition rate of the system that makes the system too slow for use in smaller hand-held ophthalmic instruments, where inadvertent relative motion between the instrument and the eye is constantly taking place as the operator aligns the instrument by hand. Moreover, the system of U.S. Pat. No. 4,881,807 and other prior art systems like it are expensive to manufacture because of the types and number of optical and opto-electronic elements necessary, and the dimensional criticality in positioning these elements with respect to one another.
Therefore, it is an object of the present invention to provide a position detection system and method for an ophthalmic instrument that is faster than prior art systems and methods to provide updated output at a higher frequency.
It is another object of the present invention to provide a position detection system for an ophthalmic instrument that is relatively inexpensive to manufacture.
It is yet another object of the present invention to provide a position detection system for an ophthalmic instrument and a method for calibrating the system, whereby the system can be calibrated easily and periodically after manufacture to ensure accuracy.
It is yet another object of the present invention to provide a position detection system for an ophthalmic instrument that cooperates with an instructive display for presenting alignment cues to an operator.
In furtherance of these and other objects, an ophthalmic instrument incorporates an afocal position detection system for determining X-Y-Z alignment status of the instrument relative to a patient""s eye. In a preferred embodiment, the position detection system comprises first and second light sources on opposite sides of a central optical axis of the instrument, and corresponding first and second light-sensitive area detectors positioned to receive light from an associated light source after it has been reflected by the cornea. The detectors provide signal information indicative of the local x-y position of an illumination spot formed thereon. In a preferred embodiment, the first and second detectors are quad-cell detectors having four quadrants, and the illumination spot size is about the size of one quadrant, whereby the x-y position can be determined based on the four signal levels generated by the quadrants. Collector lenses after each light source and in front of each detector minimize vergence in the light beam as it illuminates the eye and as it arrives at a detector.
The local x-y data from each detector are then provided as input to a series of stored geometrical relationships determined during instrument calibration for giving the X-Y-Z global alignment status of the instrument relative to the eye. The geometrical relationships are multiple regression equations for X, Y, and Z, wherein regression coefficients for each equation are determined by reading local x-y data from the detectors for an artificial eye placed at a plurality of known X-Y-Z positions during calibration. The regression coefficients are stored during calibration and used during normal instrument operation to quickly calculate X, Y and Z coordinates based on local x-y data from the detectors as an operator positions the instrument relative to a patient""s eye.
A xe2x80x9cheads-upxe2x80x9d display is preferably connected to receive the X-Y-Z position data and provide instructional cues to the operator for moving the instrument to achieve alignment. In a current embodiment, the heads-up display comprises a polar array of light emitting diodes selectively illuminated to indicate a desired X-Y movement direction, and a linear array of light emitting diodes selectively illuminated to indicate a desired Z movement direction. An image of the heads-up display is presented to the operator along the instrument optical axis through the use of a beamsplitter that allows a macro-image of the patient""s eye to be transmitted as well along the optical axis, whereby the X-Y polar array is arranged circumferentially about the macro-image.